The present disclosure relates to methods and systems for performing color separation.
Color printing has traditionally required the integration of many unique and varied talents to see a project through from conception to a printed page. Prior to the advent of “desktop publishing,” ideas or concepts were typically first drawn by hand and photographed, any text or illustrations added, and the aggregate of pictures and text used to produce a printed page. The traditional process generally required, in addition to design personnel, a paste-up person, typesetting bureau and a lithography department that would produce separations from the photographs. Desktop publishing has relieved some of the burden of publishers by allowing color production, i.e., drawing and layout, to be integrated electronically using personal computers. Color documents can now be designed, enhanced, color-corrected, and separated on a desktop computer.
The computer can output data to a wide variety of output devices. Output devices such as laser printers, plotters, image-setters, and other printing devices produce an image or “visual representation” on a sheet of paper, a printing plate or the like. A printing device can print dots on a piece of paper corresponding to the information of a bitmap or pixelmap (where a pixelmap is characterized as having a depth of two or more bits). A “raster” printing device creates a visual representation by printing an array of pixels arranged in rows and columns from the bitmap.
A desktop printer uses toner to produce the colors comprising a color publication. Because composite printing is generally efficient only for small quantity jobs, larger quantities of the same publication are generally reproduced on a commercial printing press using ink. The printing press advantageously prints multiple pages of printed matter at the same time onto a single sheet. Other devices used for printing multiple pages at the same time include image setters and plate setters.
Methods of printing color publications using a commercial printer press include process-color printing, spot-color printing, or a combination of the two. Process-color printing separates the original image into its cyan (C), magenta (M), yellow (Y), and black (K) components to recreate the original shadings of color in the publication. This is accomplished by printing dots of the process-color inks in different combinations in close proximity to simulate a variety of colors on a printed page. Spot-color printing involves printing one or more specific colors (or inks) that have been specified according to a color matching system. One popular color matching system is the PANTONE MATCHING SYSTEM by Pantone, Inc. Spot-color printing is often used to produce colors that are not easily produced using CMYK inks, such as silver, gold, and fluorescent colors. Spot-color printing is also used in lieu of printing the four process colors, such as when only a few types of inks are required for a particular publication.
Before a color publication can be reproduced on a commercial printing press, each page containing composite art must be separated into its component colors by printing a separation for each ink (cyan, magenta, yellow, and black, if process colors are to be printed) and any spot colors. Spot-color printing requires a separation for each color being printed. A commercial printer uses film separations to create the printing plates used on the press. For instance, if one specifies the four process colors and a single spot color in a publication, there will be five separations, and hence printing plates, for each page. A separate component ink is added by each plate as the pages in the publication pass through the press. A more detailed explanation of the commercial printing process is discussed in the Commercial Printing Guide from PageMaker, Version 5.0. For additional information on desktop publishing generally, see Desktop Publishing in Color by Michael Kieran, published by Bantam Books (1991).
The printing devices typically produce output in response to a page description language (PDL) input. The PDL is a high level language for describing objects to be displayed by an output device. A printing device may receive page description data in the form of a PDL file. In order to produce a finished sheet, a printing device typically interprets all of the objects contained in the PDL file. Thereafter, the printing device renders objects within the data into bitmaps and prints the sheet. Depending on the printing device, printing of a band may begin as soon as the objects within that band are rendered. Alternatively, printing may not begin until all of the objects for the sheet have been rendered.
One popular PDL is called PDF (Portable Document Format) which was originally created by Adobe Systems Incorporated (San Jose, Calif.) as a way of electronically exchanging and reviewing documents. The PDF format retains the original color and style of any printable document and allows it to be viewed on any computer platform, including mainframes, servers, workstations, and personal computers. This format has become widely accepted as a standard for viewing on-line documents in their original format, without the restrictions of standard web pages.
A PDF file contains a PDF document and other supporting data. The PDF document contains one or more pages. Each page in the document may contain any combination of text, graphics, and images in a device- and resolution-independent format. This is the page description. A PDF document may also contain information possible only in an electronic representation, such as hypertext links, sound, and movies. In addition to a document, a PDF file contains the version of the PDF specification used in the file and information about the location of important structures in the file.
A PDF page content stream contains objects with color information. For example, each PDF object has a fill and a stroke color associated with the object and any of these colors may be defined as a colored pattern associated with the object. The pattern contains color information as part of the pattern description. Color information about objects is represented in a composite manner, i.e., the objects (as filled in by the patterns) contain a composite color information for the object. The pattern itself does not fully describe the object it is associated with. For example, the geometry of the object is not described by the pattern.
PDF defines a type of color known as a pattern, discussed on page 287 of the Adobe Portable Document Format Reference Manual version 1.3 (Mar. 11, 1999). There are two major types of patterns in PDF. The first type is called a tiling pattern; the second is called a smooth shading pattern. Tiling patterns are procedural; they include a sequence of explicit marking instructions, which are repeated once per tile. Smooth shading patterns are declarative; they include a description of the desired effect in terms of transitions between colors across a certain area. Like forms, patterns can contain sequences of marking operations; they are the same as patterns in the PostScript language. The space in which a pattern is defined is pattern space. The transformation from pattern space to user space is specified by a matrix contained in the pattern.
In addition to patterns, PDF supports named resources such as XObjects. PDF supports three types of XObjects: images, forms, and pass-through PostScript language fragments, among others. A form is a self-contained description of any text, graphics, or sampled images that may be drawn on several pages or more than once on a single page. A Form XObject is specified by a PDF stream, which is a contents stream.
Due to the popularity of the PDF format, it is desirable for printing devices to support the ability to directly print documents from a PDF file in various processes. These processes may vary from printing per plate, printing per spot color, among others. Moreover, certain printing processes require the separation of colors for each object as the object is drawn on zero or more of the plates. For example, given a four component color space (e.g. CMYK) and the existence of a color component for an object in each of the four color spaces, the object needs to be drawn up to four times, one time for each color component associated with a printing plate.
High-quality printing, such as that required by the publishing industry, poses many difficult problems in controlling the separations. For example, color printing is compromised if paper is not properly aligned as it makes multiple passes through the plates of a printer. Given the composite construct of a PDF document, the process of separating various components of a page to be printed in a given ink can be resource intensive and time consuming.